The Added R-Value of insulation is not the same as the Material R-value when bulk insulation is installed between the joists. The Added R-Value is is reduced by thermal bridging at the joists where the insulation level is reduced or in the case of batts missing entirely. This creates thermal bridges (or short circuits) where the Added R-Value will be considerably lower than the Material R-value. These calculations are for timber joists where thermal resistance Rj is still significant.
¹Steel joists may have significantly poorer performance especially if they are not covered. The problems with steel joists are that they are connected to steel truss and the area in contact with the ceiling is much larger than the cross secion used in these calculations. These factors combined with the high thermal conducivity mean that effective R values may be significantly lower than those produced by these calculations. The calculations also presume that the section will be filled with thermal insulation as is the case with loose fill.
Loose-fill ________________________ In roof air film = 0.11 K·m²/W ~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~| Loose-fill insulation covering [X] [X] [X] ~| | 95 mm high joists ======================== ----- Plaster + air film = 0.17 K·m²/W |_| | 48 mm wide joists at |_______| 600 mm centres Batts ________________________ In roof air film = 0.11 K·m²/W : :~~~~~: :~~~~~: :~~~~~ ~~~~| Batt insulation over, but not covering [X] [X] [X] ~| | 95 mm high joists ======================== ----- Plaster + air film = 0.17 K·m²/W |_| | 48 mm wide joists at |_______| 600 mm centres
|Material||Ceiling System R-Value||Added R-Value||Tile Roof System w/o RFL||Tile Roof System with RFL|
|R Value (R)||Equiv. Batt||Loose Fill (Rl)||Batts (Rb)||Loose Fill (Rl-Rs)||Batts (Rb-Rs)||Loose Fill (Rl+Rt)||Batts (Rb+Rt)||Loose Fill (Rl+Rf)||Batts (Rb+Rf)|
See the further reduction in system R-value if we put in downlights that are not covered.
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